The present invention relates to a reflective liquid crystal display device, particularly relates to a reflective liquid crystal display device having a feature in a light reflection plate thereof.
In the reflective liquid crystal display device, a light having been incident from outside is reflected by a light reflecting plate provided inside of the liquid crystal display device to use the reflected light as a light source for the display, for which reason any back light is not necessary for the light source. It has been considered that the reflective liquid crystal display device with the light reflecting plate is suitable for reductions in dissipation power, thickness and weight rather than a light transmission liquid crystal display. The reflective liquid crystal display device includes a liquid crystal layer, a switching element such as a thin film transistor or a diode, and a light reflecting plate. The liquid crystal layer may comprise either a twisted nematic liquid crystal, a super-twisted nematic liquid crystal, a guest-host liquid crystal or a polymer dispersion liquid crystal.
The reflective liquid crystal display device is required to have a bright and white displaying performance in the light transmission mode. The light reflective performance of the light reflecting plate depends upon various parameters of the surface roughness of the light reflection plate, particularly sloped angles of convex and concave portions constituting the rough surface of the light reflection plate, and the irregularity of the surface roughness.
In the conventional reflective liquid crystal display device, an active matrix driving system is used together with the light reflection plate, wherein thin-film transistors (TFT) or diodes having a metal/insulator/metal structure, for short an MIM structure, are used as a switching element for realizing high line and high quality pictures.
The structure of the conventional reflective liquid crystal display device will be described with reference to FIG. 1.
The conventional liquid crystal display device comprises top and bottom substrates 1 and 4 which sandwich a liquid crystal layer 11. The top substrate 1 comprises laminations of a glass substrate 2 and a transparent electrode 3 which is in contact with the liquid crystal layer 11. The bottom substrate 4 comprises a glass substrate 5, arrays of thin film transistors 6 with an inverse stagger structure provided on the glass substrate 5, a polyamide interlayer insulator 7 provided to cover the arrays of thin film transistors 6 and a light reflection plate 10 provided over the polyimide interlayer insulator 7 and under the liquid crystal layer 11. The light reflection plate 10 has a surface roughness 14. The light reflection plate 10 is made of a conductive material so that the light reflection plate 10 serves as a pixel electrode. The surface of the polyimide interlayer insulator 7 has a roughness 18 on which the light reflection plate 10 substantially corrugated to have the surface roughness 14 is provided. The polyimide interlayer insulator 7 has contact holes 49 over drain electrodes 9 of the thin film transistors 6 so that the light reflection plate 10 extends not only over the polyimide interlayer insulator 7 but also within the contact holes 49 whereby the light reflection plate 10 made of a conductive material for serving as the pixel electrode is in contact with the drain electrodes 9 of the thin film transistors 6.
The liquid crystal layer 11 comprises a guest-host liquid crystal which has been injected into a gap between the top and bottom substrates 1 and 4.
An incident light 12 is transmitted through the glass substrate 2, the transparent electrode 3 and the liquid crystal layer 3 to the light reflection plate 10 by which the transmitted light is reflected and transmitted through the liquid crystal layer 3 to the substrate 1 and then outputted therefrom. The reflective liquid crystal display device utilizes the reflected light 13. In order to obtain a sufficient brightness of the screen for the liquid crystal display, it is necessary that lights having been incident in various angles may be reflected in a direction just or almost vertical to the surface of the substrates 1 and 4. The rough surface 14 of the light reflection plate 10 comprises convex and concave portions which form top and valley portions and sloped portions. If the incident light having been in the direction vertical to the surfaces of the substrates 1 and 4 is reflected at the top and valley portions of the rough surface 14 of the light reflection plate 10, then the reflected light is transmitted in the direction just or almost vertical to the surfaces of the substrates 1 and 4. If the incident light having been in a direction tilted from the normal of the surfaces of the substrates 1 and 4 is reflected at the sloped portions of the rough surface 14 of the light reflection plate 10, then the reflected light is also transmitted in the direction just or almost vertical to the surfaces of the substrates 1 and 4. The rough surface 14 including the top and valley portions and the sloped portions allows the incident lights having been incident not only in the vertical direction but also in the tilted direction to be reflected and transmitted in just or almost the vertical direction to the surfaces of the substrates 1 and 4.
The above conventional reflective liquid crystal display device may be fabricated as follows. The descriptions of the fabrication processes for the above reflective liquid crystal display device will hereinafter be made with reference to FIGS. 2A through 2G.
With reference to FIG. 2A, a gate electrode 15 is formed on the glass substrate 5.
With reference to FIG. 2B, a gate insulation film 16 is formed, which extends over the glass substrate 5 and the gate electrode 15. A surface of the gate insulation film 16 has a hillock over the gate electrode 15. A semiconductor layer 17 being doped or undoped with an impurity is formed on an entire surface of the gate insulation film 16. A surface of the semiconductor layer 17 also has a hillock over the hillock of the gate insulation film 16.
With reference to FIG. 2C, the semiconductor layer 17 is selectively removed by patterning process to leave the same over and in the vicinity of the hillock of the gate insulation film 16.
With reference to FIG. 2D, source and drain electrodes 8 and 9 are formed, wherein the source electrode 8 extends over a left side portion of the remaining semiconductor layer 17 and over the gate insulation film 16 in the vicinity of the left side portion of the remaining semiconductor layer 17, whilst the drain electrode 9 extends over a right side portion of the remaining semiconductor layer 17 and over the gate insulation film 16 in the vicinity of the right side portion of the remaining semiconductor layer 17 to thereby form a thin film transistor 6.
With reference to FIG. 2E, a polyimide insulation film 7 is formed, which extends over the gate insulation film 16 and over the source and drain electrodes 8 and 9. A surface of the polyimide insulation film 7 is subjected to a patterning to form a rough surface which comprises convex and concave portions whereby the rough surface comprises top and valley portions and sloped portions.
With reference to FIG. 2F, a contact hole 49 is formed in the polyimide insulation film over the drain electrode 9 to expose a part of the drain electrode 9.
With reference to FIG. 2G, a light reflection plate 10 made of a conductive material is formed on the rough surface with the convex and concave portions of the polyimide insulation film 7 as well as in the contact hole 49 so that the light reflection plate 10 is in contact with the drain electrode 9 of the thin film transistor 6. The light reflection plate 10 is patterned to form a pixel electrode.
In the above fabrication processes, the photo-lithography processes have been used six times. The first photo-lithography process was made in patterning the gate electrode 15. The second photo-lithography process was made in patterning the semiconductor layer 17. The third photo-lithography process was made in forming the source and drain electrodes 8 and 9. The fourth photo-lithography process was made in forming the rough surface 14 of the polyimide insulation film 7. The fifth photo-lithography process was made in forming the contact hole 49. The sixth photo-lithography process was made in patterning the light reflection plate 10.
The above fabrication processes are disclosed in Tohru Koizumi and Tatsuo Uchida, Proceedings of the SID, Vol. 29, 157, 1988.
The above fabrication processes for the conventional reflective liquid crystal display device comprises a number of the photo-lithography processes and complicated steps. Particularly, the three photo-lithography processes are needed to form the thin film transistor 6 as a switching device and further three photo-lithography processes are needed to form the light reflection plate 10. Those facts result in increase in the manufacturing cost and this increase raises a problem with a high price of the reflective liquid crystal display device.
In the above circumstance, it had been required to provide an improved reflective liquid crystal display device at a low price and an improved fabrication process for the improved reflective liquid crystal display device at a low manufacturing cost.